1. Field of the Invention
The present invention relates to an audio control method and an audio control apparatus which controls an audio system equipped with a plurality of amplifiers that perform amplification of audio data.
2. Description of the Related Art
A widespread of digital broadcasting in recent years has made it possible for people to enjoy multichannel acoustic sounds. In advanced digital satellite broadcasting, a 22.2-channel multi-sound system is proposed in addition to current 2-channel and 5.1-channel surround sound systems. The 22.2-channel multi-sound system is composed of a group of speakers including an upper layer of nine channels, a middle layer often channels and a lower layer of three channels, which are arranged in a vertical direction, and 2 channels of LFEs (Low Frequency Effects) which are placed on a floor surface.
Upper layer speakers are used for the purposes of localizing a sound image at locations above an audience or listener, and expressing an early reflected sound and a rear reverberant sound. Moreover, the upper layer speakers can express the vertical movement of the sound image by mutual use of the middle layer and the lower layer. Middle layer speakers can reproduce the most main sound sources, and can express acoustic sounds common to existing multichannels such as 2 channels, 5.1 channels and 7.1 channels, and so on. Lower layer speakers are used for localizing a sound image below the audience. For example, the lower layer speakers are used to express sound sources at a lower portion of a screen, such as the sound of footsteps, the sound of a stream on a river surface, etc. In addition, the 2 channel LFEs are low-frequency effects channels for generally reproducing a low-frequency component of 120 Hz or less, and are used for expressing the feeling of sound spreading, etc.
Multichannel sound can reproduce a higher-precision sound space by means of speakers with different frequency characteristics together with their arrangement. Here, human aural characteristics will be considered. The frequencies of sound which humans can perceive are said to be from 20 Hz to 20 kHz. However, the volume of sound which humans feel changes with frequency. Human conversations are conducted in the frequency range of 200-8,000 Hz, and human sensitivity is the highest in the range of 1,000-3,500 Hz. That is, a sound can be heard at different volume levels if its frequency varies, even with the same sound intensity. This means that even in the human audible frequency range, a sound in a low frequency area or in a high frequency area can not be heard unless it has a certain amount of sound volume.
While multichannel audio can provide listeners with a high sense of realism or a high-quality sound effect, it increases the number of reproduction speakers, and accordingly the number of amplifiers required. In addition, in multichannel audio, a wide range of tones can be expressed, but sound in a frequency band of unclear acoustic effects is subjected to output processing, depending on the volume level thereof, so there arises a problem of increasing power consumption.
Japanese patent application laid-open No. 2004-343414 describes a technique that suppresses power consumption by controlling switching power supplies which supply electric power to amplifiers. In this Japanese patent application laid-open No. 2004-343414, the power consumption is intended to be reduced by controlling a switching frequency according to a set value of the volume level of sound of an audio output. Specifically, as the set value of the volume level becomes smaller, the switching frequency is accordingly lowered, whereby a loss due to a switching operation is reduced, thus suppressing the consumption of electric power.
However, the technique described in Japanese patent application laid-open No. 2004-343414 is on the premise of a 2-channel sound system, and no consideration is given to the reduction of power consumption in a multi-channel sound system of three or more channels.
A 5.1-channel surround sound system and a 7.1-channel multichannel sound system, which make use of reflected sound and reverberant sound and give effects according to respective frequency bands, are different in the volume level of reproduced sound for which acoustic efficiency can be verified or perceived. In these sound systems, power consumption cannot be reduced in an efficient manner only by performing the same switching frequency control as in a 2-channel sound system. That is, in order to achieve the efficient use of electric power while obtaining an optimal acoustic efficiency, it becomes important to control the number of channels of audio output.